It is well-known that measurement, characterization, and testing of electronic circuits are necessary aspects of developing, debugging, and manufacturing electronic products. Typically a test probe is used to make electrical contact with one or more terminals of an electronic circuit, and provide an electrical pathway between that electronic circuit and externally deployed Measurement, characterization, and/or testing equipment. For products that operate at high speeds, it is also known that the impedance of the test probe becomes an important parameter.
It is highly desirable to measure the effective value of passive components (e.g., chip capacitors, inductors, resistors) both at specific frequencies and over specific frequency bands for which those components are being considered. Component manufacturers' data sheets typically publish impedance or component values at only certain low frequencies, typically in the 1 to 100 MHz range, and very seldom at 1 GHz. The effective values of such components at high frequencies, i.e., above 100 MHz, and particularly in the GHz range, vary from their low frequency values due to package parasitics, lead inductance, and fringe capacitance. The users of such components need to measure the effective values these components present to a circuit at specific design frequencies, or over the desired frequency band. High frequency impedance measurement systems are typically very expensive and range in the tens of thousand of dollars. Most RF and microwave designers often use network or vector analyzers that are widely used in the RF and microwave engineering labs, and design their own makeshift tools by stripping the end of a microwave test cable and soldering the center conductor to a circuit board where the target component can be mounted and tested. To make the measurements somewhat accurate, the tip of the test cable must be calibrated. The calibration process often involves a laborious and time consuming process of soldering and de-soldering the cable tip several times to terminate the center conductor into a 50 ohm or 75 ohm load, short and open circuits. Alternatively, some designers use four individual sections of microwave test cables of equal length with each section having a type N or SMA connector on one end, and stripped center conductor at the opposite end. A first section is soldered to a printed circuit board and having its stripped center conductor terminated into a 50 ohm or 75 ohm resistive load previously mounted on a printed circuit board. A second section is soldered to a second circuit board and its stripped center conductor tip is shorted circuited to ground. The third section is soldered to a third circuit board with its stripped center conductor soldered to a small open circuited microstrip transmission line. These three sections now provide the calibration set. A common long test cable is then used to connect a network analyzer to each of these three cable-board assemblies for calibration. A fourth section is soldered to a fourth printed circuit board where the component or device under test will be soldered for an impedance measurement.
In addition to the testing and/or characterization of components, vendor-supplied RF and microwave circuits, and modules such as filters, attenuators, voltage controlled oscillators, amplifiers, an so on, are almost always tested and evaluated for performance verification and qualification by engineering personnel and design engineers before such circuits and devices are used in new product design. Often large sample quantities are tested. Further, in production and quality control, these devices are also often tested. For testing, circuits and devices as well as modules are normally placed on printed circuit boards, and RF connectors are soldered at input and output ports to provide external interfaces for connection to test cables.
Prototype and engineering circuits require RF connectors at input and output ports. These connectors, which are relatively expensive especially in small quantities, must be soldered to the printed circuit boards. If the boards are damaged or circuits need to be redesigned which is often the case during prototype stage, RF connectors cannot often be salvaged and after several soldering cycles, removing, and mounting again to multiple printed circuit boards they are rendered useless, or no longer reliable for future use. Once RF connectors such as SMAs are soldered to a circuit board, they can easily be damaged when they are removed from the circuit board by overheating the solder joints.
What is needed are low-cost, high frequency passive probe assemblies that are operable to be calibrated without consuming valuable RF connectors in the process of calibration.